Calliope should be able to handle time series with different resolutions

[Tim Tröndle]

When time series have different resolutions, Calliope fails with a Time series indices do not match between ts1.csv and ts2.csv error message, even if resample and subset_time are turned on. I find that behaviour rather surprising.

I suppose the reason for this behaviour is that there is no central time authority -- but rather Calliope expects all time series to have the same indices. As a default I guess this is fine, but maybe we could change the behaviour when resample is turned on?

Here's a proposal:

We could move subset_time into resample. By default, resample is turned off and all time series are expected with the exact same scope and resolution. Whenever resample is on, subset_time must be set as well: both parameters combined are a complete definition of the temporal scope and resolution. The only checks Calliope does in this case are:

1. Are all timeseries within scope?
2. Is the resolution of all timeseries higher or equal to the new?

Steps to reproduce the problem

Run a model with at least two time series having different resolution.

Calliope version

0.6.5

[Stefan Pfenninger]

Should be dealt with in tandem with #356